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Full text of "Handbook Of Chemical Engineering - I"

REFRACTORIES                                        499
Unless these clays are low in suck constituents they are apt to fuse at too low a tern-
e is   to exl
is   to exclude the more refractories
Analyses of typical siliceous fireclays are:
I/ESTER,                 GROSS ALMERODE,
, ABK.                      GERMANT, PER
PERCENT                         CENT
^lica; .................................. 72.05                     73.08
Abmma ................................  17.21                     15.75
Titanium oxide ............................                      2 10
Ferric oxide ........................... '_    i25                      0.62
Lime ....................................    0.45                      0.54
Magnesia ...............................    0.75                      0.33
Potash ...............................                                    0 27
Soda .........................................                      0.16
Loss on ignition ..........................    8,55                      7 jj
Refractory Clay Products. From the standpoint of the manufacturer the clay refractories may be subdivided into the following divisions : (1) Flint-clay refra-torios; (2) siliceous-clay refractories; (3) plastic-clay refractories; (4) grog refractories.
Flint-clay Refractories.  The flint-clay refractories consist essentially of a predominating amount of flint clay, as high as 85 per cent in the best qualities, with a smaller admixture of plastic fireclay, usually just sufficient for bonding purposes. In Europe no flint clays are available and hence clays of the plastic type as well as kaolins are used exclusively. It is important to use as small an amount as possible of plastic clay in the bonding of flint clay except when the former is of exceptional quality, since after all it is the small amount of more fusible material corresponding to the eutectic which governs the refractoriness of the whole mass. It would be desirable in certain instances to grind together the plastic ('lay with part of the flint clay so as to increase the refractoriness of the bonding material. An ideal combination would consist of flint clay bonded together with weathered plastic clay of the same type, but usually such a material is not available. Another very satisfactory composition would be a mixture composed of flint clay and plastic kaolin, like the somewhat impure top clay overlying the kaolin deposits of Georgia.
The. boat-resisting qualities of this class of refractories are decreased, of course, with decrease in flint clay and mixtures containing as low as 20 or 30 per cent of this material and the balance of plastic fire clay cannot be expected to be more refractory than the hitler. In the case of flint clays which show a large contraction in firing it may be necessary to calcine a portion, either in the regular down draft or in shaft kilns, but the practice is avoided as much as possible in the making of standard bricks. For tiles and larger shapes, however, the use of some calcined "hard" clay is often necessary.
Firebricks containing much flint clay are manufactured by grinding the clay mixture in dry pans, screening and tempering or pugging it with water in a pug mill or wet pans. The bricks are molded either by hand or in so-called soft mud machines. It is possible also to press such mixtures into bricks in the damp state, using for this purpose powerful presses. Bricks thus made are said to be "dry-pressed" and are considerably heavier than those made from the soft-plastic mixture. They also possess physical properties decidedly different from those of the soft mud brick,